Igneous and metamorphic processes in the Shap Granite and its aureole

Abstract

The Shap Granite outcrops in eastern Cumbria, N.W.
England and is a post—orogenic granite intruded during the
Lower Devonian (ie 394 Ma) into rocks of Ordovician to
Siturian age. It is of adamellite composition and is notable
in having megacrysts of orthoclase which crystallised late
(relative to the matrix) by an essentially metasomatic
process. Late in its cooling history, hydrothermal fluids
resulted in alteration ano mineralisation in and about the
joint structures. Comparison of whole — rock element and
stable isotope data between the hydrothermally — altered and
non —altered granites shows that this late — stage process- was isochemical and only involved localised redistribution of the available elements, the hydrothermal fluid being derived from the granite itself and acting in a closed system.
The associated suite of granitic dykes extends the
chemical range of Shap granitic rocks from the restricted
compositions of the granite itsetf (around 68%)to 62 to 77%
Si02. Mineralogical and chemical evidence suggests that the
Shap magma compositions were controlled mainly by biotite
and plagioclase fractionation over much of this silica
range.
The granite is intruded about much of its outcrop into
rocks of the mid—Ordovician, calk—alkaline Borrowdatt
Volcanic Group. The aureate is lkm wide and 'generally
displays limited contact metamorphic reactions apart from in
the Blue Quarry, where higher sub—surface heat flow during
the granite's intrusion resulted in localised more extreme
metamorphic and some metasomatic conditions resulting in the
development of garnet veins. Analysis of 61 samples from the
aureole region show only very limited modifications to their
original calk—alkaline chemistry. Stable isotope and the
whole rock geochemistry indicates that the contact
metamorphism was essentialty, isochemical and that chemical
interaction with the granite did not occur. Aureole
metamorphic reactions took place at moderate temperatures
(250 — 400°C) apart from in a narrow (<1000 zone about the
granite contact where temperatures may have reached close to
those of the granite-solidus (around 600°C).
Xenoliths from the 'granite Pink quarries at Shap contain the same mineralogy as their host granite, including
the megacryst orthoclase. The xenolith chemistry compares
well with that of the more basic dykes, with the xenoliths -
forming a coherent group over the compositional range 56 to
674 Sia. Comparison with the country—rock chemistry shows
that the xenoliths were not externally derived but probably
represent quenched, more basic, comaqmatic melts related to
the generation of the main Shap Granite magma. Limited Sr
and Nd isotope data for the xenoliths show that these
isotopes were in equilibrium with the granite and lends
strong support for the cogeneric origins and chemical
relationships for the granites, aykes and xenoliths
together.
The narrow aureole width is typical of those formed by
conductive ratner convective cooling with interaction with
the adjacent granite limited to thermal effects and not
involving the cycling of fluids through the granite or
aureole as in convective pluton cooling.